With the gradual enhancement on the requirements of capacity and flexibility of the optical communication system, the coherent optical communication technology has become more and more important. In comparison with incoherent technology (such as on-off key, OOK) or self-coherent technology (such as differential quadrature phase-shift keying, DQPSK), the coherent technology has the following advantages: optical signal-to-noise ratio (OSNR) gain of approximately 3 dB; the capability to use more efficient modulation technologies (such as quadrature modulation, QAM) to enhance transmission capacity, and the capabilities to make convenient use of electric equalization technology in response to channel change, and lower production cost, etc. Coherent reception requires that the frequency and phase of the local oscillation be identical with the phase of the carrier wave, that is to say, the difference in phases (phase offset) and difference in frequencies (frequency offset) between the local oscillator and the carrier wave should be zero, for otherwise phase offset and frequency offset not being zero would appear in the received base band signal to thereby greatly affect the performance of the coherent receiver. In an electric coherent receiver, the phase offset and frequency offset are removed by means of a phase locked loop that precisely controls the phase and frequency of the local oscillation. However, insofar as optical coherent reception is concerned, due to restrictions of such factors as the technologies and standardization of optical devices, it is very difficult to implement a phase locked loop in the receiver to carry out precise control of the local oscillator laser. There is hence a need in the optical coherent receiver for a new technique to remove the influences brought to the performance of the receiver by the frequency offset and phase offset of the base band signal. There have currently been certain technologies in application to compensate the phase offset in the base band signal, but a precondition for proper operation of these technologies is the extremely low frequency offset (to the level of MHz).
FIG. 1 shows an optical coherent receiver employing a phase locked loop. In FIG. 1 a phase offset detector 106, a loop filter 105 and a controller 104 constitute the phase locked loop. The output 111 of the phase locked loop controls a local oscillator laser 103 to make consistent the frequency and phase of the output 102 of the local oscillation laser 103 with the frequency and phase of the carrier wave. In addition, an optical frequency mixer 107, and photoelectric detectors (PD) 108 and 109 in FIG. 1 constitute a front end processor for converting an inputted optical signal 101 into a base band electric signal. A data recover 110 recovers data signal. Due to restrictions of such factors as the technologies and criteria of optical devices, it is very difficult to realize the receiver as shown in FIG. 1. Accordingly, some technologies have been proposed to process the base band digital signal so as to remove the phase offset.
FIG. 2 shows an optical coherent receiver employing digital phase recovery technology. In comparison with FIG. 1, no control is performed in FIG. 2 to the local oscillation laser 103, instead, a digital phase recover 204 is employed to remove the phase offset between the local oscillation and the carrier wave. The digital phase recover 204 averages the phase offsets of a plurality of continuous symbols to remove noise so as to obtain the actual phase offset, and then makes use of this phase offset to correct the phases of these symbols so as to remove the phase offset. In FIG. 2, an optical frequency mixer 107, photoelectric detectors 108 and 109, and analog-to-digital converters 201 and 202 constitute a front end processor for converting an optical signal into a base band digital electric signal 203. As can be seen from the foregoing explanation, the working principle of the digital phase recover is to regard the phase offset of the plurality of continuous symbols as a constant number. However, when the frequency offset is not zero, the phase offset of the symbols varies with variations in time. Consequently, when the frequency offset is greater than tens of MHz, the digital phase recover cannot operate normally. Whereas in an actual optical communication system, the frequency offset might be as high as several GHz due to the influences of such factors as temperature. Accordingly, frequency offset compensation is indispensable in any optical coherent receiver employing the digital phase recovery technology.
FIG. 2 further shows an example of a prior art digital phase recover. However, the digital phase recover can also be realized by other devices as long as they can recover digital phase. As shown in the lower portion of FIG. 2, the input of the digital phase recover is the base band electric signal 203 outputted by the front end processor of the coherent receiver, namely, I+jQ=exp(jθd+jθ). In general cases, the base band electric signal 203 contains not only data information θd but also phase offset θ between the carrier wave and the local oscillator. The base band electric signal 203 is firstly inputted to an argument calculator 207 to obtain an argument 208. The argument 208 is respectively inputted to a subtracter 223 and a subtracter 210. A feedback phase difference before N symbols is subtracted from the argument 208 at the subtracter 210 to coarsely obtain the data phase, and then inputted to a 2π modulo calculator 211 and a quotient rounder 212. The 2π modulo calculator performs a 2π modulo calculation on the signal and restricts it to between 0 and 2π, and the quotient rounder divides the signal having been restricted to between 0 and 2π by the 2π modulo calculator by a predetermined value (such as π/2), and rounds up the integral portion. The signal 203 is inputted to phase offset complex value extracting sections 214, 215, 216 or 217 in accordance with the result of the quotient rounder 212. The phase offset complex value extracting sections obtain a real part w2 and a imaginary part w1 of a phase offset √{square root over (2)} exp(jθ) represented by complex numbers as shown by the formula in FIG. 2, for example.
Noise in the estimated imaginary part and real part is removed by a noise removing device 219. The noise removing device can for instance be realized by an averager. The averager is a simple device that performs mathematical averaging on the N symbols, and can be used to remove noise. Subsequently, the argument calculator 221 obtains the argument θ of the averaged complex number w2+jw1=√{square root over (2)}(cos(θ)+j sin(θ))=√{square root over (2)} exp(jθ), and outputs the same, namely a phase offset 222. The phase offset 222 is subtracted from the signal 208 at the subtracter 223 and outputted as the recovered digital phase. At the same time, the phase offset 222 is further supplied to a subtracter 209 for the next round of application after passing through a delaying device 213 to have been delayed for N symbols.
To make it easy for description, the subtracter 223 can be regarded as a phase recover, and the remaining component parts (namely the argument calculator 207, the subtracter 210, the 2π modulo calculator 211, the quotient rounder 212, the phase offset complex value extracting sections 214-217, the noise removing device 219 and the argument calculator 221, etc.) can be regarded as phase offset calculating sections.